1. Field of the Invention
The present invention relates generally to a method and apparatus using optical interference, and more particularly to an improved method and apparatus using optical interference to measure fly height (FH).
2. Description of the Prior Art
“Fly height (FH)” is the vertical distance between a slider and a disk in a disk drive system or hard disk drive (HDD). FH is measured after calibration and an error is introduced during this process, which is clearly undesirably.
That is, in a HDD, a slider carries a read/write element that flies over a disk having magnetic layers with a given speed. Accurate measurement of the FH is important and need be controlled to know, for example, what signal is received at a given FH and for controlling the process by being able to measure FH repetitively. FH can be measured at different points of the slider so as to capture tilts associated with the slider. A mean plane is calculated by the average of different FH measurements of the slider. The read gap is the distance between the disk and the read element of the slider and the write gap is the distance between the write element of the slider and the disk. The air bearing surface (ABS) is the surface facing the disk. The slider flies at a certain pitch angle and to measure the latter, the FH is measured at one or more points on the ABS close to the leading edge of the slider (the edge upstream of the airflow) and another one or more points close to the trailing edge (the edge downstream of the airflow) and the difference between the two (or more) FH measurements divided by the planar distance between these points is the calculated pitch angle. Similarly, the slider flies at a certain roll angle which is calculated by measuring the FH at one or more points on each side of the ABS, but this time the points are in the radial direction relative to the disk.
When defining the magnetic spacing or the distance between the read and/or write element and the magnetic layers on the disk, it is necessary to know the precise FH. As the magnetic spacing is reduced to obtain higher density, the accuracy of the FH becomes more critical. Fly heights are measured during manufacturing and more specifically, during a calibration process. This is done to compensate for a pre-existing error. That is, the distance between the disk and the slider, i.e. FH, is presumed to be and remain parallel or even in terms of all of the points of the slider are at the same distance away from like points of the disk. However, this presumption is flawed because the slider is actually tilted relative to the disk. Measurements of the FH are performed, during calibration, assuming a parallel position of the slider relative to the disk and the result of the calibration is then used to measure the tilt and fed back into the measurement.
During calibration, the intensity of reflected light is measured and the calibration result is in turn used to measure the FH at normal disk velocity and slider skew angles.
One prior art method of measuring FH uses optical interference where an optical FH tester is used to measure FH during calibration. The slider is flown over a transparent surface. Then light of a given wavelength is directed to interfere at the slider-disk interface and the reflected intensity is then a direct function of the air gap thickness, which is the FH.
The principle of operation of an optical FH tester, for example the DFHT5 tester made by KLA-Tencor of San Jose, Calif., is used to measure a reflected intensity and to use a calibration curve to translate the intensity to a FH, i.e. air gap thickness. The calibration curve is typically obtained by retracting the slider from the disk and measuring the maximum of intensity when the light interferes constructively (at ¼ wavelength) and the minimum of intensity when the light interferes destructively (at ½ wavelength). These two data points are then used to scale the intensity axis of the theoretical Int=f(FH) curve, wherein Int is the intensity and f(FH) is a function of the FH. The theoretical curve is computed using the optical indices of the slider and disk (n,k).
During calibration, the maximum and minimum intensity in the Red, Green and Blue are typically as follows:
BlueGreenRedFirst maximum points:105 nanometer (nm)130 nm155 nmFirst minimum points:220 nm270 nm315 nmSecond maximum points:330 nm410 nm480 nmFIG. 1 shows a graph of a typical Int=f(FH) curve for two different slider pitch values. The x-axis represents the FH (in nm) and the y-axis is the normalized Int (in percentage). The spot size of the light source is typically 25 micrometers (um) in diameter. Thus, the reflected intensity is the average over the spot size. When the slider is retracted from the disk, the air bearing breaks and typically, the maximum-respectively minimum of intensity is found when the FH is around 100 nm for blue light, respectively 220 nm. At those FH, the pitch of the slider can be as high as 3000 micro radians (urad). It is typically expected to be 500-1000 urad. The precise value can be vary with the air bearing surface (ABS) design, each slider sample, and the retraction conditions (pull off velocity and disk speed (in revolutions per minute (rpm)).
The problem with the foregoing measurement of FH is that with a pitch angle (of the slider) of a given value, such as 1000 micro rads, the FH at one end of the spot size is 25 nanometers higher than the other side, i.e. 1000 urad×25 micrometers=25 nanometers. Therefore, the maximum intensity measured during calibration is smaller than if the pitch were exactly zero. For the same reason, the minimum intensity is higher than if the pitch were zero. One consequence of the foregoing is that the instrument can easily report a “negative FH”, which is clearly unacceptable. The measured intensity changes and an undesirable error is included in FH. At lower FH and/or higher pitch differences between measurement and calibration, the error becomes larger. The problem of roll is the same problem as with pitch. The pitch angle results from the movement of the slider from the trailing edge to the leading edge and the roll angle results from movement of the slider from one side to another side.
Thus, when using white light as an illumination source, temporal coherence is lost for successively longer wavelengths until interference effects are no longer observed as the separation increases. This limits the largest spacing that can be measured using the interference of light method to measure FH. Typically, films that are thicker than about 1,250 nm cannot be measured. Accordingly, the maximum pitch and roll angles that can be measured are also limited.
Thus, there is a need for a method and apparatus to improve the accuracy of FH measurement by removing an error introduced during calibration into the FH measurement.